Foams based on expandable polystyrene particles have achieved great industrial importance as thermal insulation and packaging material. They are produced on an industrial scale by first preparing expandable styrene polymers by suspension polymerization of styrene in the presence of a blowing agent, foaming these by heating to form foam particles and subsequently welding the latter together in molds to produce shaped articles.
Polystyrene foams are rigid foams. Their low elasticity is a disadvantage for many applications, for example in the packaging sector, since the packaged goods can be only insufficiently protected against impact and shock and the foam moldings used as packaging material break at only a small deformation.
Attempts have therefore already been made in the past to increase the elasticity of polystyrene foams.
WO 94/25516 relates to prefoamed beads of rubber-modified styrene polymers having cell walls comprising a polymer matrix comprising a) a continuous polystyrene phase and b) flat rubber particles having a l':d' ratio of from 10:1 to 70:1 dispersed therein, where the individual particles have a rubber shell in which one or more polystyrene cores are enclosed. To prepare these prefoamed beads, polybutadiene or a polystyrene-polybutadiene two-block copolymer is dissolved in styrene and the styrene is polymerized. This gives a rubber-modified styrene polymer containing rubber particles which have a diameter of from 0.1 to 1 .mu.m and in which the core-shell structure is preformed. This rubber-modified styrene polymer is impregnated with blowing agent and granulated, and the beads containing blowing agent are foamed. However, the toughness of foams made up in this way still leaves something to be desired; in particular, the elastic recovery is unsatisfactory.